I-beams are used in residential and commercial construction as the joists in ceilings and floors, often instead of more conventional rectangular sawn lumber joists, such as 2-by-12's. An I-beam is a beam that includes what are called flanges as the top and bottom of the "I," and what is called a web as the body of the I, between the top and bottom flanges. The strength of an I-beam depends on what it is made of, what shape it has, and how well its parts are attached to each other. For example, an I-beam made of steel is usually stronger than the same beam made of wood, and an I-beam with a tall web usually is stronger than a beam with a short web made with the same size flanges and same thickness of web.
An I-beam used in a floor or ceiling is often selected based on how much the beam flexes or moves when it is in use. A beam may move a lot without breaking, so that a floor made with this beam might not collapse, but might move so much that it feels springy, making it very awkward for anyone walking or sitting on the floor, and can cause its holding nails to loosen and squeak. A bouncing or squeaking floor is disturbing to those both above and below the floor. Thus, a good I-beam is strong enough not to flex or squeak excessively. For floors and ceilings in occupied areas, an acceptable amount of movement is generally less than 1/360th of the span. The span is the distance the beam extends without any support. For a 10-foot span, this means the beam can only flex about 1/3-inch at any point on the beam.
When an I-beam is flexed under a load, some parts of the beam are being squeezed under compression, and other parts are being pulled under tension. The flanges are under the most compression or tension because they are being squeezed by or pulled along the web as it is bent into a curved shape. The taller the web, the more this squeezing or pulling acts on the flanges for a given amount of bending of the web, which is why taller I-beams are stronger than shorter ones. The technical term describing this is the moment of inertia of the beam, which expresses the ability of a beam to resist flexing. The higher the moment of inertia, the more a beam resists flexing. In an I-beam, the combination of the web and the flanges creates a beam with a relatively high moment of inertia, even though the moment of inertia of the web or flanges, separately, is relatively low.
Steel I-beams can be extruded out of a single piece of material, in much the same way as children's clay is pressed through an I-shaped hole to form a long I-shaped piece. The same could be done with wood by cutting the I-beam from a single, solid piece of wood, but this would be very wasteful of the wood. Furthermore, wood and other wood-based materials often have different strengths in different directions. Thus, wood-based I-beams are made from several separate pieces that are glued, nailed or pressed together. These beams are called "built-up I-beams" because they are built from several different pieces of material.
One example of a known built-up I-beam is manufactured by Trus Joist MacMillan a Limited Partnership of Boise, Id., and is disclosed in U.S. Pat. No. 4,893,961. This beam is made from a web of plywood or oriented strand board (OSB) and flanges of laminated strand lumber (LSL) or laminated veneer lumber (LVL). A groove or rout is cut into the lower or upper face of each flange, and the flanges are glued to the web by forcing the web into the rout in each flange. While the dimensions can vary, one such I-beam with an overall height of 117/8-inches is made with a web that is 7/16-inches thick by 101/2-inches high, and matching flanges that are 11/2-inches thick by 25/16-inches wide. The rout bisects the width of each flange and penetrates to about half of the thickness of the flange, so that the web extends about half the way through each flange.
Plywood, OSB, LSL, and LVL are part of a broad range of manmade lumber materials referred to as engineered lumber. The advantages of using engineered lumber for I-beams include the general uniformity of the material, resulting in more predictable structural performance of the beam, and the availability of high quality engineered lumber of the needed dimensions compared to the availability of conventional sawn lumber of the same dimensions. Other types of engineered lumber, including parallel strand lumber (PSL), glued laminated timber (GLT) and particleboard have varying degrees of applicability to I-beams.
The distinguishing factors between the above-mentioned types of engineered lumber generally involve the types, sizes and relative orientations of fiber used, the types and proportions of adhesives used, and the methods of forming the fiber and adhesive into a finished product. OSB, as used herein, refers only to engineered lumber incorporating selectively oriented strands of wood fiber that are bonded with adhesive cured in a hot platen press. The press is normally of a fixed size, operating in a batch process, but may also be a continuously operating belt-type press. Actually, when dealing with structural components other than the web of an I-beam, the proper terminology is "oriented strand lumber" and not "oriented strand board." Therefore, oriented strand lumber or OSL will be used to describe this oriented strand product bonded with adhesive cured in a hot platen press. But because some still may refer to this product as oriented strand board or OSB, those terms should be considered herein to be synonymous with oriented strand lumber or OSL.
OSL is distinguished from LSL by OSL's hot platen press, as opposed to LSL's steam injection press. OSL is similarly distinguishable from PSL by PSL's unheated press that utilizes microwave energy to cure the adhesive instead of hot platens. However, OSL as used herein does encompass materials that may include fibers and adhesives similar to those used in LSL or PSL, provided the fibers and adhesives are formed into a finished product in a hot platen press. The remaining types of engineered lumber are made with fiber that is too short to provide the strength of strands, such as is found in particleboard, or too long to be processed as a strand, such as is found in plywood, LVL and GLT.
While the above-described OSB/LVL I-beam provides an adequate beam for most applications, there is an interest in the market for built-up beams with flanges made of materials other than LVL or LSL. However, simply replacing the LVL flanges in the Trus Joist MacMillan I-beam with flanges made of OSL does not provide a satisfactory beam. The combination of the distances traditionally spanned and the loads carried, particularly on longer spans, results in several structural inadequacies for an I-beam made with known OSL flanges.
One such inadequacy results because OSL is generally made in a batch process, in which adhesive and strands of wood fiber are mixed and placed in a press of a defined length to make panels of the desired thickness. The length is normally 24-feet, shorter than is required for many applications for built-up beams. While it is possible to join the edges of such panels with a finger joint to create a panel longer than 24-feet, finger joints often are not so strong as the remaining length of the OSL panel. Thus, the finger joint can create a point of failure. A similar problem can result because there are occasionally localized density variations in the OSL, such as a suboptimal concentration of adhesive relative to wood fiber, so that the OSL flange has weak points.
Another inadequacy results because of a density variation that occurs across the thickness of OSL made using hot platen press technology. A much higher density is found at the outside or skin of OSL than is found in the center or core. This means that the skin is harder than the core. Typically, the skin has a density of about 45-pounds per cubic inch and the core has a density of about 30-pounds per cubic inch. LSL and PSL do not have this density variation, and thus their use in beam flanges does not present the same technical problems as does the use of OSL in beam flanges.
When OSL is placed under a sufficiently high compression load, such as when the lower flange of an I-beam rests on a wall, the OSL may fail by crushing. The low density core crushes under a lower load than the high density outer skin. Typically, the core of thicker OSL crushes under lower loads than does the core of thinner OSL made with the same fibers and adhesives. OSL flanges should be about 11/2-inches in thickness if they are to properly hold nails and other fasteners used to attach floors or ceilings to the beam. It has been found that OSL of this thickness tends to crush too easily to be used in many installations in which an I-beam joist is desired.
This crushing is accentuated by the use of a rout in the flanges, because the thickness of the web bears primarily against the low-density core of the OSL. The rout is used to improve the adhesion of the flange to the web, so non-routed flanges are not the solution to the problem addressed by the present invention. The angle of the rout could also be increased to broaden the flare of the rout, so that more of the compression is carried by the walls of the rout as opposed to the bottom of the rout. However, this would decrease the grip of the rout on the web as well, so this too is not the solution to the problem addressed.
Yet another drawback with using OSL flanges is the cost of OSL of the required thickness of 11/2-inches. The cost of an OSL panel increases at a rate about proportional to the square of the thickness for most currently available manufacturing processes. Accordingly, 11/2-inch-thick OSL is approximately four times as expensive as 3/4-inch-thick OSL.